This invention relates to a converter for an antenna to receive signals from two satellites, which is preferable for conversion of received signals generated by a receiving antenna into received signals for transmission, when the receiving antenna receives radio waves transmitted from two artificial satellites being located in the same direction seen from a receiving point.
Heretofore, a broadcasting satellite (so-called BS) and a communication satellite (so-called CS), both of which transmit radio waves that can be received in Japan, have been located in different directions seen from a receiving point. Also, a system of each satellite for transmission of radio waves is different from each other. More specifically, radio waves transmitted from the broadcasting satellite are right-hand circularly polarized waves, while radio waves transmitted from the communication satellite are two orthogonal polarized waves each consisting of a vertically polarized wave and a horizontally polarized wave. Consequently, such radio waves transmitted from either of the artificial satellites (i.e., BS or CS) have been received by the respective receiving antennas constituted independently of each other.
In the near future, however, a new communication satellite (specifically, NSAT-N-SAT110, produced by Nippon Satellite Systems, Co., Ltd. and Space Communication, Co., Ltd.) is going to be launched to be located in the same direction as the broadcasting satellite, seen from the receiving point in Japan, more specifically, at 110 degrees of east longitude. In addition, it has been decided that two types of circularly polarized waves, that is, right-hand and left-hand circularly polarized waves, should be adopted as radio waves to be transmitted from the new communication satellite.
Under such circumstances, it will become possible to receive radio waves transmitted from the broadcasting satellite as well as those transmitted from the foregoing communication satellite to be newly launched by a single receiving antenna, by improving a receiving antenna in conventional use for reception of radio waves transmitted from the broadcasting satellite.
A radio wave transmitted from the foregoing new communication satellite has a frequency higher than that of a radio wave transmitted from the broadcasting satellite, and therefore, the frequencies of received signals from the two satellites do not overlap each other. However, a radio wave of a right-hand circularly polarized wave and a radio wave of a left-hand circularly polarized wave, both transmitted from the foregoing new communication satellite, should be in the same frequency band to effectively utilize the assigned frequencies, in the same manner as the other communication satellite.
For this reason, in the event that, as mentioned above, radio waves transmitted from the two artificial satellites (i.e., BS and CS), being located in the same direction seen from the receiving point, are received by a single receiving antenna, and then, received signals generated by the receiving antenna are converted into received signals for transmission having frequencies lower than those of the transmitted radio waves using a common converter (or frequency changer), it is necessary to switch the received signal(s) to be inputted to the converter from/to a BS received signal of a right-hand circularly polarized wave as well as a CS received signal of a right-hand circularly polarized wave (hereinafter, also referred to as a CS (R) signal) to/from a CS received signal of a left-hand circularly polarized wave (hereinafter, also referred to as a CS (L) signal) according to necessity.
With such structure, however, in cases where the received signals are transmitted to a plurality of receiving terminals, for example, in a home community receiving system, a desired broadcast can not be received at each of the plurality of receiving terminals.
In view of the above problem, this invention has been developed. An object of the invention is to provide a converter for changing frequencies of received signals generated by a single receiving antenna, when it receives radio waves transmitted from two artificial satellites (i.e., BS and CS) located in the same direction seen from a receiving point, to convert the received signals into received signals for transmission, the converter being capable of simultaneously transmitting the received signals after frequency changes to a terminal side by way of a single transmission line.
In order to attain this object, the invention as recited in claim 1 provides a converter for an antenna to receive signals from two satellites, which is provided in a receiving antenna capable of receiving: (1) a radio wave of a right-hand circularly polarized wave within a first frequency band, transmitted from a first artificial satellite;
(2) a radio wave of a right-hand circularly polarized wave within a second frequency band in which frequencies are higher than those of the first frequency band, transmitted from a second artificial satellite; and (3) a radio wave of a left-hand circularly polarized wave within the second frequency band, transmitted from the second artificial satellite, the first and second artificial satellites being located in the same direction seen from a receiving point, the converter being capable of converting a received signal resulting from each of the radio waves received by the receiving antenna into a received signal within a predetermined transmission frequency band, having a frequency lower than that of the respective radio wave and being able to be simultaneously transmitted, together with received signals resulting from the other radio waves, via a single transmission line. The converter according to the invention operates as follows:
A right-hand frequency changing portion changes frequencies of the received signals of the right-hand circularly polarized waves within the first and second frequency bands to convert them into received signals within the respectively predetermined transmission frequency bands, using first oscillating signals generated by a first oscillator. On the other hand, a left-hand frequency changing portion changes a frequency of the received signal of the left-hand circularly polarized wave within the second frequency band to convert it into a received signal within a transmission frequency band in which frequencies are higher than those of the received signals outputted from the right-hand frequency changing portion, using a second oscillating signal generated by a second oscillator having an oscillation frequency lower than that of the first oscillator. Subsequently, the received signals from the right-hand frequency changing portion are transmitted to a received signals output portion, via a first filtering portion for cutting off at least signal components in the same frequency band as the received signal outputted from the left-hand frequency changing portion falls within. The received signal from the left-hand frequency changing portion is transmitted to the received signals output portion, via a second filtering portion for cutting off at least signal components in the same frequency bands as the received signals outputted from the right-hand frequency changing portion fall within. The received signals transmitted from the right-hand and left-hand frequency changing portions are mixed at the received signals output portion, and then outputted from a common output-terminal.
Consequently, by use of a converter for an antenna to receive signals from two satellites according to the present invention, received signals from a receiving antenna for receiving the aforementioned radio waves (1) to (3) can be converted into received signals for transmission which can simultaneously be transmitted to a terminal side via a single transmission line. The converter is therefore preferable for transmission to a plurality of receiving terminals of received signals from the aforementioned two artificial satellites, for example, in a home community receiving system.
Now, the oscillation frequency of the second oscillator may be set such that, by a frequency-changing operation of the left-hand frequency changing portion, the frequency of the received signal outputted from the left-hand frequency changing portion becomes higher than those of the received signals outputted from the right-hand frequency changing portion. However, if the oscillation frequency of the second oscillator is set to be too low, a transmission frequency of the received signal outputted from the left-hand frequency changing portion becomes high. As a result, a transmission loss in transmission of the received signal to the terminal side becomes great. In addition, there is a possibility that transmission equipment, such as a transmission line for received signals and a booster provided on such a transmission line, having conventional frequency characteristics can not be applied.
For the above reasons, the oscillation frequency of the second oscillator is desirably set such that a difference between the lowest frequency of the received signal outputted from the left-hand frequency changing portion and the highest frequency of the received signals outputted from the right-hand frequency changing portion is as small as possible in the range that the difference is equal to or over a frequency (for example, 50 MHz) with which the received signals outputted from the right-hand and left-hand frequency changing portions can be separated from each other by the first and second filtering portions. More specifically, as recited in claim 2, the oscillation frequency of the second oscillator should be set such that, by operation of the left-hand frequency changing portion, a signal component within the first frequency band becomes a signal component within a transmission frequency band overlapping those of the received signals outputted from the right-hand frequency changing portion.
Also, according to the present invention, the radio wave of the right-hand circularly polarized wave and the radio wave of the left-hand circularly polarized wave, both received by the receiving antenna, are respectively changed in frequencies by means of two oscillators, that is, the first and second oscillators. Consequently, a possibility arises that a harmonic component of a difference between the oscillation frequencies of those oscillators might be overlapped with the received signals.
For the above reason, as recited in claim 3, it is desirable that the oscillation frequencies of the first and second oscillators should be set such that the harmonic component of the difference between the oscillation frequency of the first oscillator and that of the second oscillator never overlaps any of the transmission frequency bands of the received signals outputted from the right-hand and left-hand frequency changing portions.